Internal combustion engines may include water injection systems that inject water into a plurality of locations, including an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Injecting water into the engine intake air may increase fuel economy and engine performance, as well as decrease engine emissions. When water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to the water. This heat transfer leads to evaporation, which results in cooling. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx, while a more efficient fuel mixture may reduce carbon monoxide and hydrocarbon emissions.
The injection of water into an engine typically includes the dispensing of water in a constant stream. However the inventors herein have recognized that such an injection may result in improper mixing of the injected water into the air path. In particular, manifold water injection may result in uneven water distribution amongst cylinders coupled to the manifold. For example, water injected upstream of a group of cylinders may not distribute evenly to each of the cylinders due to evaporation, mixing, and entrainment issues, in addition to the airflow maldistribution among cylinders. Further, due to differences in architecture of the engine (e.g., differences in the location, size, and arrangement of intake runners of cylinders within a cylinder group), maldistribution of water amongst the cylinders may occur. Further still, maldistribution of water may occur due to differences in the angle of the manifold water injector upstream of a group of cylinders relative to each runner. If the angle of the water injector or the arrangement of the runner is such that a portion of the injected water puddles, then the water injection benefits of that portion of the injected water may be lost. As a result, uneven charge cooling may be provided to the engine cylinders. In some cases, this may aggravate any existing cylinder-to-cylinder imbalance (e.g., due to air-to-Fuel ratio imbalance, coolant temperature maldistribution, etc). Overall, the maldistribution can result in the full potential of the water injection not being realized (for example, due to the full extent of charge air cooling not being achieved).
In one example, the above issues may be at least partly addressed by a method for an engine comprising: injecting water into an engine intake manifold as a plurality of pulses from a water injector, the pulsing adjusted with reference to intake valve timing based on output from an intake manifold oxygen sensor. In this way, cylinder-to-cylinder water injection imbalances may be better learned and compensated for.
As an example, during conditions when water injection is enabled, such as at high engine loads, water may be injected into an engine intake manifold as a plurality of uniform, evenly spaced pulses whose phasing coincides with the intake valve opening timing of the engine cylinders receiving the injected water. Based on knock sensor output following the injecting, cylinder-to-cylinder variations in water distribution may be inferred. For example, imbalance may be inferred due to different knock intensities in each cylinder following the common water injection. The cylinder-to-cylinder variations in water distribution may be due to variations in the transport delay between the location of water injection and the individual cylinders, which in turn may be based on, as an example, differences in geometry between the runners or water injectors of the individual cylinders. To learn the imbalance, water may be pulsed into the engine intake manifold with a phasing based on the intake valve opening timing of the engine cylinders and further based on their learned knock intensities. Further, based on a deviation between an expected manifold dilution following the injecting relative to an actual dilution (as inferred based on the output of an intake oxygen sensor), individual cylinder transport delays may be learned. The transport delays may then be compensated for during a subsequent water injection by adjusting a timing and amount of the phasing of the individual water pulses. For example, water injection amounts may be increased to compensate for potential water puddles, while water injection timing may be advanced to compensate for water transport lags.
In this way, maldistribution of water between engine cylinders may be better quantified and compensated for. The technical effect of relying on a change in the output of an intake oxygen sensor following a water injection to estimate the maldistribution is that a time and amount of change in engine dilution can be correlated with transport delays to specific cylinders. As a result, air, fuel, and water to the corresponding cylinder can be appropriately adjusted to reduce knock issues and improve the cooling effect and dilution effect of the water injection. Overall, the benefits of water injection may be extended over a wider range of engine operating conditions, improving engine efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.